Add option to skip bundle

I loath having all of my gems installed globally, I want to only have the
things I _really_ need being globally accessible and then modifying my
`GEM_HOME` and `GEM_PATH` depending on the project I'm in.

To make that work with setting up a new rails project,
I always run `--skip-bundle` and install my gems later on.

This patch just mimics this argument from rails itself: https://github.com/rails/rails/blob/ef99d4cd3ecc58a8c1484740b2fb5447dbda23ab/railties/lib/rails/generators/app_base.rb#L32-33
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